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jonny3010
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what resistor do I need?

Thu Nov 22, 2012 8:00 pm

what resistor do I need?

i got a 5v volt plug 4A output. What resistor do i need to limit an device to 1A so it cant draw anymore than 1A. I have 4 devices that need 1 Amp Each so a resistor is the best one to limit the power so they don't get over amped

Devices
Raspberry pi
Routor
Phone cable for charging
Hard drive

So 1amp resistance to each device?
Last edited by jonny3010 on Thu Nov 22, 2012 8:06 pm, edited 2 times in total.

techpaul
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Re: what resistor do I resistor?

Thu Nov 22, 2012 8:01 pm

jonny3010 wrote:what resistor do I need?
How long is a piece of string?

You have not said for what puropses and in what application.
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or http://www.pcserviceselectronics.co.uk/pi/

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jonny3010
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Re: what resistor do I resistor?

Thu Nov 22, 2012 8:07 pm

techpaul wrote:
jonny3010 wrote:what resistor do I need?
How long is a piece of string?

You have not said for what puropses and in what application.
updated the post i press enter and it sent it off before i was finnished

techpaul
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Re: what resistor do I need?

Thu Nov 22, 2012 8:09 pm

jonny3010 wrote:what resistor do I need?

i got a 5v volt plug 4A output. What resistor do i need to limit an device to 1A so it cant draw anymore than 1A. I have 4 devices that need 1 Amp Each so a resistor is the best one to limit the power so they don't get over amped

Devices
Raspberry pi
Routor
Phone cable for charging
Hard drive
You do NOT need a resistor the 5V will be less than 5V at the device.

For power distribution just like the fuse box in your house you need 1A or less fuses in line with each device which will have a lower voltage drop.

All resistors will drop voltage as defined by Ohms law

V = I * R

So for any current flowing through the resistor, a voltage drop will occur.

So if 1A and 1ohm you will drop 1V, so the device will only get 4V
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jonny3010
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Re: what resistor do I need?

Thu Nov 22, 2012 8:11 pm

so how can i limit the amps going to the phone/routor/pi/HDD so it does not get all the 4Amps and only 1Amp or am i wrong a resisters are not needed? and im thinking of over volting?

techpaul
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Re: what resistor do I need?

Thu Nov 22, 2012 8:21 pm

jonny3010 wrote:so how can i limit the amps going to the phone so it does not get all the 4Amps and only 1Amp
Devices draw as much current as they need, unless the supply cannot provide that much current.

Fuses and other protection are to protect against faults and blowing the supply or other parts causing fire.

You can only really limit current by devices like fuses or cutoffs. Anything else is MUCH more complicated. Consider the wiring in your house, what limits any outlet to JUST the power needed for that device. It does not you only stop the COMBINED load being too high.
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jonny3010
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Re: what resistor do I need?

Thu Nov 22, 2012 8:23 pm

ok i understand now. so a 4AMP will not make my phone charge any fast than a normal 1AMP does. and wont make the phone blow up if i use a 4AMP supply

Grendell
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Re: what resistor do I need?

Wed Nov 28, 2012 10:06 am

Hi
Just spotted your post.

Please make sure the supply is a REGULATED 5volts. An unregulated supply able to give 4 amps could well produce 7v or 8v when only lightly loaded. Borrow a voltmeter to check 5v DC.

If it is regulated at 5v then a resistor will not be a good way to limit the current. A 5R resistor will, it is true limit the short circuit current to 1amp, however, if your Pi is trying to draw 1/2 an amp the resistor will drop your 5v to around 2 1/2v. In fact ANY current will drop the 5V to a lower voltage.

v drop = Current (A) x Resistance (ohms)

As mentioned a (glass) fuse would provide "short circuit" protection as will a solid state Poly-Fuse. Both will have a tiny voltage drop, the latter will also reset when the short circuit is removed.

Hope this helps

G

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jonny3010
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Re: what resistor do I need?

Sat Dec 01, 2012 9:42 pm

I tested the plug with a volt meter and i got 5.125v and i think its regulated. i got it for £14 (LINK) i hope it works well for my pi

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